U.S, allied sources caution that their assessment is preliminary, and they're seeking conclusive proof; senior U.S. Democrat urges Obama to order air strikes against Syria.

U.S. and allied intelligence agencies' have made a preliminary assessment that chemical weapons were used by Syrian forces in an attack near Damascus this week, likely with high-level approval from the government of President Bashar Assad, according to American and European security sources.

The early intelligence finding could increase pressure for action by President Barack Obama, who has made clear that he plans to tread cautiously even as his aides air their differences in a debate over possible military responses to the Syrian government.

The intelligence sources, speaking on condition of anonymity, cautioned that the assessment was preliminary and, at this stage, they were still seeking conclusive proof, which could take days, weeks or even longer to gather.

Soon following the release of the assessment, the top Democrat on the House of Representatives Foreign Affairs Committee, Eliot Engel, urged Obama on Friday to order air strikes against Assad's government.

Engel cited Obama's statement that the use of chemical weapons by Assad's forces would cross a "red line" and cause the United States to act to halt such violations of international law.

"If we, in concert with our allies, do not respond to Assad's murderous uses of weapons of mass destruction, malevolent countries and bad actors around the world will see a green light where one was never intended," Engel wrote in a letter to Obama and obtained by Reuters.

In his first public comments since Wednesday's attack in the Damascus suburbs, Obama called the incident "very troublesome" and a "big event of grave concern" but made clear he was in no rush to get war-weary Americans entangled in another Middle East conflict.

The commander may have been under pressure from top echelons to clear rebels from the area and protect his own troops against rebel counter-attacks, the source said.

"What we've seen indicates that this is clearly a big event, of grave concern," Obama said in an interview on CNN's "New Day" program that aired on Friday, as anti-Assad rebels braved the front lines around Damascus to smuggle tissue samples to UN inspectors from victims of Wednesday's apparent mass poisoning.

But when pressed about his comment a year ago that chemical weapons use in Syria would cross a "red line," Obama - who was on a two-day bus tour of the Northeast - expressed caution.

At Thursday's White House meeting, which lasted more than three hours, Obama's aides had a "robust discussion" of the diplomatic and military options available to the president, U.S. officials said.

Among the military options are targeted cruise missile strikes on Syrian military units believed responsible for chemical attacks or on Assad's air force and ballistic missile sites. Seen as more risky would be a sustained air campaign against Assad's forces, such as the one used in Libya in 2011.

A White House spokesman reiterated Obama's position that he did not intend to put "boots on the ground," and an administration official said Thursday's White House meeting also steered clear of the idea of enforcing a "no-fly" zone there, the official said.